I am thinking about making a 0 degree down quilt, designed to go into a bivy sack. Weight limit is 2 pounds. I am thinking 4inches of loft. I am 5 foot 10 about 190 lbs. Can it be done? Don't have the bivy sack yet, but looking at the ID Salathe bivy sack.

Yep. Tim Marshall did it for me. Mine is 20.8 ounces. I've only taken it down to about 20° F and I weighed 235 lbs at the time…heavier than I designed the quilt for, so I wasn't as good at preventing drafts as I would have liked.

Nunatak sells a quilt they rate to 5* that weighs in at 30oz in Quantum, so it can be done. I'd worry about the bivy creating condensation though. If you watch this video essay about a snowshoe around Crater Lake, I believe I recall the BPL who used a bivy/sleeping bag as his shelter system ended up with quite a bit of condensation collected in his bag after 3 days.

Probably better to sacrifice oz and design the quilt wide enough that drafts are not an issue(in a tent, of course)

I have never made any gear to date, looking at the link I am
surprised that the fabric weight added up to 10oz.

I know condensation can add up I have a 6 moon designs cape
that is also a tarp/tent that is a pound and most likely will carry that if the forecast is for rain, that way I can keep the top of the bivy open to breath, and be able to button up if needed.

By the way the integral designs Salathe, not the lightest bivy
but it does have extra features to keep more dry, the middle of the bivy can open up and fold down during good weather or under a tarp also in worse weather on the sides you can open up under the zipper flaps and get cross ventilation. I think it's design is better than every other bivy I have seen so far in regards of ventilation. Of course I still would expect
moisture build up over a few days, especially if buttoned up. I need the ability to set up camp in tight spots, as a climber, so this choice of system.

ive got a microbivy .. not a salathe … so that this with a grain of salt

i find some condensation on the inside the times ive tried it … nothing big, but it is concentrated around the opening … sometimes even if yr careful, youll breath into yr bag when sleeping especially if its cold